Single-Site Iron(III) Centers
A R T I C L E S
possible state of iron in FeZSM-5. Although there is no
consensus on the nature of the iron centers, it seems clear that
some type of isolated iron site plays an important role in the
selective processes catalyzed by FeZSM-5.
Scheme 1
Given the increasing interest in the structure and catalytic
chemistry of supported iron centers, we have attempted to
develop reliable routes to stable, well-defined inorganic iron
species bound to an oxide support. Herein, we describe a process
for the introduction of isolated, tetrahedral iron(III) centers onto
a silica surface, involving grafting reactions with molecular
precursors. This method produces iron sites that are remarkably
stable with respect to thermal degradation to iron oxide clusters.
Previous reports have described attempts to synthesize iron-
supported silica materials from inorganic iron species such as
FeCl3,1
4,15
FePO4, and Fe2(SO4)3‚7H2O. In general, these
16
17
procedures do not produce stable, isolated iron surface species,
as the aqueous media and calcination treatments lead to
condensation of the iron into iron oxide clusters.
Other efforts to prepare catalytic, supported iron species have
involved the grafting of molecular iron complexes onto silica
via an organic “tether”. Many of the resulting heterogeneous
catalysts have been inspired by iron-containing enzymes (es-
As discussed elsewhere, we have shown that the thermolytic
3
7-45
molecular precursor route to oxide materials
used to introduce catalytic titanium sites onto the surface of a
may also be
18-24
pecially cytochrome P-450 and methane monooxygenase),
2
5-36
46,47
and involve grafting of biomimetic iron model complexes
silica support.
A potential advantage to this approach is that
onto silica. For example, tetrakis(pentafluorophenyl)porphyrin
iron(II) has been grafted onto silica gel modified with 1,6-
it may allow molecular-level control over the structure of the
catalytic site, as illustrated in the generalized (hypothetical)
transformations shown in Scheme 1. Initially, the precursor
molecule is bonded to the surface via protonolysis reactions,
33
1
1
3+
diaminohexane, and [Fe2O(η -H2O)(η -OAc)(TPA)2] (TPA
tris[(2-pyridyl)methyl]amine) may be incorporated into silica
)
34
t
modified with poly(ethylene oxide) and poly(propylene oxide).
which in the case of a precursor of the type M[OSi(O Bu) ] ,
3
n
t
t
Molecular iron species have also been attached to the surface
of an MCM-41 silica material via the linker 3-aminopropyltri-
may occur with loss of HO Bu or HOSi(O Bu) . Depending on
3
this grafting chemistry, the species bound to the surface will
35
methoxysilane, and iron porphyrin species have been incor-
be attached via M-O-(surface) or Si-O-(surface) linkages,
respectively. Calcination should then lead to loss of the organic
fragments of the immobilized species, in a manner similar to
that observed for the “bulk transformations”, to produce isolated
MOx‚(n-1)SiO2 species on the oxide surface. The new site may
be partially supported by the few equivalents of silica that are
introduced by the molecular precursor, and in this way,
stabilized. The goals of the work described here are to determine
whether this method is effective for the introduction of stable,
single-iron sites on silica and to characterize the properties of
such species as catalysts for selective hydrocarbon oxidations.
porated into MCM-41.36
(
14) Schuchardt, U.; Pereira, R.; Krahembuhl, C. E. Z.; Pufo, M.; Buffon, R.
Appl. Catal. 1995, 131, 135-141.
(
15) Tagawa, T.; Seo, Y.; Goto, S. J. Mol. Catal. 1993, 78, 201-210.
16) McCormick, R. L.; Alptekin, G. O.; Williamson, D. L.; Ohno, T. R. Top.
Catal. 2000, 10, 115-122.
(
(
(
17) Kurusu, Y.; Neckers, D. C. J. Org. Chem. 1991, 56, 1981-1983.
18) Cytochrome P450; Ortiz de Montellano, P. R., Ed.; Plenum: New York,
1
986.
(
19) Fontecave, M.; M e´ nage, S.; Duboc-Toia, C. Coord. Chem. ReV. 1998, 178-
0, 1555-1572.
20) Rosenzweig, A. C.; Frederick, C. A.; Lippard. S. J.; Nordlund, P. Nature
8
(
1
993, 366, 537-543.
(
(
21) Feig, A.; Lippard, S. J. Chem. ReV. 1994, 96, 759-805.
22) Rosenzweig, A. C.; Nordlund, P.; Takahara, P. M.; Frederick, C. A.;
Lippard, S. J. Chem. Biol. 1995, 2, 409-418.
t
For this purpose, the iron precursor Fe[OSi(O Bu)3]3(THF) has
been employed. As the oxide support, we have chosen SBA-
(
23) Holm, R. H.; Kennepohl, P.; Solomon, E. I. Chem. ReV. 1996, 96, 2239-
48
1
5
as it possesses a high surface area and hydroxyl groups on
2
314.
(
24) Solomon, E. I.; Brunold, T. C.; Davis, M. I.; Kemsley, J. N.; Lee, S.-K.;
Lehnert, N.; Neese, F.; Skulan, A. J.; Yang, Y.-S.; Zhou, J. Chem. ReV.
(36) Schunemann, V.; Trautwein, A. X.; Rietjens, I. M. C. M.; Boersma, M.
G.; Veeger, C.; Mandon, D.; Weiss, R.; Bahl, K.; Colapietro, C.; Piech,
M.; Austin, R. N. Inorg. Chem. 1999, 38, 4901-4905.
(37) Terry, K. W.; Tilley, T. D. Chem. Mater. 1991, 3, 1001-1003.
(38) Terry, K. W.; Lugmair, C. G.; Gantzel, P. K.; Tilley, T. D. Chem. Mater.
1996, 8, 274-280.
(39) Su, K.; Tilley, T. D. Chem. Mater. 1997, 9, 588-595.
(40) Su, K.; Tilley, T. D.; Sailor, M. J. J. Am. Chem. Soc. 1996, 118, 3459-
3468.
2
000, 100, 235-349.
(
25) Costas, M.; Pohde, J.-U.; Stubna, A.; Ho, R. Y. N.; Quaroni, L.; M u¨ nck,
E.; Que, L., Jr. J. Am. Chem. Soc. 2001, 123, 12931-12932.
(
(
26) Chen, K.; Que, L., Jr. J. Am. Chem. Soc. 2001, 123, 6327-6337.
27) Dolphin, D.; Traylor, T. G.; Xie, L. Y. Acc. Chem. Res. 1997, 30, 251-
2
59.
(
(
(
(
28) Metalloporphyrins in Catalytic Oxidations; Sheldon, R. A., Ed.; Marcel-
Dekker: New York, 1994.
29) MacBeth, C. E.; Golombek, A. P.; Young, V. G.; Yang, C.; Kuczera, K.;
Hendrich, M. P.; Borovik, A. S. Science 2000, 289, 938-941.
30) Grinstaff, M. W.; Hill, M. G.; Labinger, J. A.; Gray, H. B. Science 1994,
(41) Terry, K. W.; Lugmair, C. G.; Tilley, T. D. J. Am. Chem. Soc. 1997, 119,
9745-9756.
(42) Coles, M. P.; Lugmair, C. G.; Terry, K. W.; Tilley, T. D. Chem. Mater.
2000, 12, 122-131.
2
64, 1311-1313.
31) White, M. C.; Doyle, A.; Jacobsen, E. N. J. Am. Chem. Soc. 2001, 123,
(43) Rulkens, R.; Male, J. L.; Terry, K. W.; Olthof, B.; Khodakov, A.; Bell, A.
T.; Iglesia, E.; Tilley, T. D. Chem. Mater. 1999, 11, 2966-2973.
(44) Kriesel, J. W.; Sander, M. S.; Tilley, T. D. AdV. Mater. 2001, 13, 331-
335.
7
194-7195.
(
(
32) Lee, D.; Lippard, S. J. Inorg. Chem. 2002, 41, 827-837.
33) Evans, S.; Smith, J. R. L. J. Chem. Soc., Perkin Trans. 2 2001, 2, 174-
1
80.
(45) Fujdala, K. L.; Tilley, T. D. J. Am. Chem. Soc. 2001, 123, 10133-10134.
(46) Jarupatrakorn, J.; Tilley, T. D. J. Am. Chem. Soc. 2002, 124, 8380-8388.
(47) Tilley, T. D. J. Mol. Catal. A: Chem. 2002, 182-183, 17.
(48) Zhao, D.; Huo, Q.; Feng, J.; Chemelka, B. F.; Stucky, G. D. J. Am. Chem.
Soc. 1998, 120, 6024-6036.
(
34) Neimann, K.; Neumann, R.; Rabion, A.; Buchanan, R. M.; Fish, R. H.
Inorg. Chem. 1999, 38, 3575-3580.
(
35) Carvalho, W. A.; Wallau, M.; Schuchardt, U. J. Mol. Catal. A: Chem.
1
999, 144, 91-99.
J. AM. CHEM. SOC.
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